I. Field of the Invention
The present invention relates to the field of packaging optical and physics-related elements of an atomic frequency standard. More particularly, this invention is directed toward a package of the optical-physics elements of an atomic frequency standard which reduces the size of the package while maintaining effective operation of the package.
II. Description of the Prior Art
Atomic frequency standards or clocks using an optical-physics package are well known. The optical-physics packages of such atomic frequency standards commonly include a microwave cavity, an absorption cell, a light source, and a light detector. In such prior art optical-physics packages, the absorption cell typically contains a vaporous material such as a rubidium isotope Rb.sup.87 which has at least one absorption line having hyperfine components, such that light having hyperfine components corresponding to the hyperfine components of this absorption line is absorbed, or at least dispersed. The separation of such hyperfine components utilized in rubidium frequency standards is due to a hyperfine transition in the ground state (5S.sub.1/2) and corresponds to a frequency of 6,834,682,612 hz. This frequency is called the natural hyperfine transition frequency.
The light source of prior art optical-physics packages preferably contains the same vaporous material as the absorption cell, such as the rubidium isotope Rb.sup.87. In this way, the light source will generate light having the hyperfine components of the absorption cell.
In such prior art devices the degree of absorption is sensed by a light detector, typically a silicon photocell, positioned on the side of the absorption cell opposite to the light source. As a beam of optical pumping light having a particular hyperfine component is absorbed, the degree of such absorption is detected as a decrease in the output of the light detector. However, this optical pumping results in a reduction in atoms having energy corresponding to that hyperfine component, causing the amount of absorption to diminish and the output of the light detector to increase to a steady state level, all of which is well-known to those skilled in the art.
The amount of absorption in such prior art devices is, therefore, additionally affected by placing the absorption cell within a microwave cavity and controlling the frequency of a magnetic field introduced into this cavity. When the magnetic field frequency is the same as the hyperfine transition frequency of the absorption cell, absorption is increased, and the output of the photocell again decreases. Moreover, when this magnetic field is varied to sweep symmetrically above and below the hyperfine transition frequency, the first harmonic output of this sweep frequency disappears from the output of the photocell. Accordingly, a phase-locked amplifier, in prior art devices, is coupled to the output of the photocell and is used to control the frequency of an oscillator which generates the magnetic field to precisely set the oscillator at the hyperfine transition frequency of the Rb.sup.87 absorption cell.
To obtain a usable signal, such prior art devices may also employ an additional uni-direction homogeneous magnetic field within the cavity; one or more buffer gases in the absorption cell and/or in the light source; and/or extremely accurate temperature control over the absorption cell and the light source.
Some prior art devices further employ a filter cell located between the light source and the absorption cell to attenuate undesirable frequencies of light generated by the light source. When vaporous Rb.sup.87 is used in the absorption cell, vaporous Rb.sup.85 may be used in the filter cell. A filter cell also provides a convenient mechanism to offset variations which may occur in the hyperfine transition frequency of the absorption cell with changes in the intensity of the light source. However, the filter cell adds substantially to the size of prior art optical-physics packages.
The internal dimensions of microwave cavities in atomic frequency standards are dictated by the hyperfine transition frequency of the vaporous material within the absorption cell and by the dielectric material within the cavity. Accordingly, for a given absorption material within the absorption cell, namely Rb.sup.87, and a given amount and type of dielectric material within the microwave cavity, i.e., the dielectric loading within the cavity, there is a given nominal resonant frequency, and therefore a given limitation on the geometric configuration of the resultant cavity. In known prior art devices, substantially the entire interior of the cavity is utilized to house the absorption cell, and the filter cell is positioned to the exterior of the cavity, interposed between the light source and the absorption cell. Moreover, since operation of a filter cell is temperature sensitive, as is operation of the absorption cell, a temperature control oven must be made large enough to house both cells, or separate ovens must be used. Accordingly, the size of the resulting optical-physics package is substantially increased by the use of a filter cell in the prior art.
A substantial advance in the prior art towards miniaturization of optical-physics packages occurred by eliminating the filter cell and instead introducing the absorption material of the filter cell, typically the isotope Rb.sup.85, into the absorption cell. The resulting hybrid absorption cell operates both as an absorption cell and as a filter cell, thereby eliminating the need for a separate filter cell and its attendant support apparatus.
Nevertheless, this advance in miniaturization led to considerable difficulties in the calibration and maintenance of such hybrid absorption cells. The choice of an absorption cell buffer gas affects the hyperfine transition frequency of the absorption cell. When separate filter and absorption cells are used, a filter cell buffer gas may be used which is different from the buffer gas used in the absorption cell. Thus, when separate filter and absorption cells are used, a filter cell buffer gas which allows for optimum use of the filter cell can be selected without any effect on the hyperfine transition frequency of the absorption cell. This option is lost in the hybrid absorption cell configuration. In addition, the hyperfine transition frequency of an absorption cell is temperature dependent, and this dependence can be controlled to some degree by the buffer gases used in the absorption cell. Obviously, in a hybrid absorption cell any selection of buffer gas to control the filtering aspects of such cell also may affect the stability and location of the hyperfine transition frequency.
It is, therefore, an object of the present invention to provide a novel miniature optical-physics package for use with an atomic frequency standard.
Another object of the present invention is to maintain the benefit of utilizing a filter cell in an optical-physics package of an atomic frequency standard without unduly increasing the size of that package.
It is still another object of the present invention to minimize frequency dependence on temperature in an optical-physics package while reducing the overall size of that package.
Additional objects and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.